APOGAMOUS FERNS. THE GENERAL PHENOMENON 

 meiosis, each with four times instead of twice the somatic chromosome number (cf. 

 Fig. 69 b, Chapter 5) . In spite of their high chromosome number such mother cells are 

 quite normal in their subsequent behaviour. It might reasonably be expected that 

 quadrivalents would be formed, since each chromosome is certainly present in quad- 

 rupUcate and all four homologues lie close together since they are quarters of the same 





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Fig. 170. Different types of sporangia in a single sorus of Cyrtomium Fortunei J.Sm. from sections. 

 a. Sixteen-celled sporangium, x 500. b. Eight-celled sporangium, x 500. c. Four-celled spor- 

 angium. X 500. d. The second abnormal mitosis in a potential four-celled sporangium by means 

 of which the chromosome number is quadrupled, x 1000. 



original chromosome (Fig. i^od). This, however, does not occur. Pairing seems to be 

 confined to sister chromosomes only, and the result is a meiosis which is as undistorted 

 in all details as is that of the eight-celled sporangia. Such sporangia when ripe contain 

 only sixteen giant spores, each with twice the nuclear content of the plant which bears 

 them. Their subsequent fate is unknown, but it is highly probable that they may be 

 responsible for some of the cases of polyploidy by simple chromosome doubling which 

 have been met with in apogamous ferns, notably in Pteris cretica (see next chapter). 



168 



